KVM: pfncache: remove KVM_GUEST_USES_PFN usage

KVM: pfncache: remove KVM_GUEST_USES_PFN usage

As noted in [1] the KVM_GUEST_USES_PFN usage flag is never set by any
callers of kvm_gpc_init(), which also makes the 'vcpu' argument redundant.
Moreover, all existing callers specify KVM_HOST_USES_PFN so the usage
check in hva_to_pfn_retry() and hence the 'usage' argument to
kvm_gpc_init() are also redundant.
Remove the pfn_cache_usage enumeration and remove the redundant arguments,
fields of struct gfn_to_hva_cache, and all the related code.

[1] https://lore.kernel.org/all/ZQiR8IpqOZrOpzHC@google.com/

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
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v8:
 - New in v8.